We are designing an ECG monitor that uses an embedded phone-modem LGA module. This phone modem is shielded, however, it is not hermetically sealed. The manufacturer requires that the board not be washed, or that a method of the user's choice be used to prevent ingress of contaminants, wash water, etc.

The same board will also have ECG circuits with high impedance differential amplifiers. The ECG circuits must avoid conductive leakage. Normal industry practice on ECG/Instrumentation boards is to always clean.

The board has several BGA processors and ASICs, uses 3 to 5 mil traces,
has 0402's, and could possibly have a need for BGAs on both primary and
secondary board sides.

What is the best way to assemble the LGA phone module onto the board in a low volume scenario, 1000 to 3000 units/month?

F.W.

Experts Comments

Given
that you really would prefer to clean due to your high-impedance circuitry, you
need to find an economical, reliable way to protect the LGA module, or
alternatively to mount the LGA module using localized soldering after cleaning
the rest of the board. Either process flow would work, however I would suggest
that the first might be more economical.

One
possibility would be encapsulating the LGA module with a moderately high
viscosity, low-modulus (elastomeric) compound after soldering and prior to
cleaning. The downsides here are:

You would need to eliminate (or plug) through-vias in the PWB
under the LGA to keep cleaning solution from getting in from the reverse side

You would then trap air under the part which would expand and
contract, exerting forces on the part and the encapsulant when heated or
cooled.

A two-step process whereby the part is underfilled and then encapsulated
could avoid the trapped-air problem, at the expense of an additional process
step and material.

Fritz ByleProcess EngineerAstronauticsFritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

My expertise is EHS, not electronic design or assembly. However, there is a
cleaning method that may be used for your module described below. That is using
carbon dioxide "pellets" (i.e. dry ice particles) to clean the module after
assembly.

This approach eliminates the use of water, and because solid CO2
sublimes into a gas, there's no residual left on the assembly. I believe the
suppliers of liquid CO2 would be a good starting point for more information on
this cleaning
method.

The use of no clean flux will create residue
opportunities that must be designed out of the assembly process. To make
this work it is not flux alone that you must worry about it is the connector
cleanliness, component cleanliness and PCB cleanliness then you have to work on
flux under the large and small components that is fully heat activated and
insulative. I would be happy to discuss these options and design risks
with you off line.

For this critical unit, the
obvious cleaning method is vapor cleaning. Vapor cleaning uses a
nonflammable hydrocarbon that is boiled to create a pure solvent vapor. The pure solvent vapor fills a chamber in a machine called a vapor degreaser
(or vapor defluxer in this case). The machine has a Vapor Trap of cooling
coils that operate around -15 F around the top of the unit.

As the parts are
lowered into the hot vapor, the condensation of the pure solvent vapor on to
the parts washes away the contaminants and they simply drip off the parts down
into the machine. A small hand held spray wand can be used to lightly
blast solvent to the underside of the BGA or FlipChip, etc.

The
parts are slowly raised up out of the pure hot vapor and they dry as they come
up to room temperature passing by the cooling coils and out of the degreaser /
defluxer.

In your specific case, there are 2 concerns. First, the solvent selected
must be compatible with the parts being cleaned. Normally we would
recommend a strong defluxing solvent such as n-propyl bromide (nPB), but in
this case it intuitively seems that you are going to have some sensitive plastic
parts on board. So, one of the mild solvents such as a fluorinated
solvent WITHOUT trans, dichloroethylene in it, would probably
suffice.

Manufacturers such as Dupont's Vertrel SMT, 3M's NOVEC HFE
7200, Petroferm's LENIUM FRA, or Solvay's Solkane HFC365mfc would be
perfect. The cleaning process for each assembly could be as fast as
90 seconds to be totally clean and dry.

Since no water is involved in this process, there should be little if any
concern with the LGA Module also being exposed to the mild fluorinated
solvent. But you could always pass this by the customer's materials
department and they can determine any issues that may exist.

The
equipment to be used could be either a small bench top vapor degreaser, such as
a Lab Kleen with a small spray wand, or a small common Baron Blakeslee
MLR120. Finally, only if necessary, the LGA module could be wrapped
up in a small teflon or nylon bag, clasped shut with teflon lacing tape or the
equivalent.

In summary, your first step is to see if your customer would accept cleaning in
a fluorinated vapor defluxing vapor, such as one of the four that I mentioned
above. Then obtain a sample or find a location to try out the
cleaning process, and then finally once proven, find the suitable equipment and
solvent to continue the process. I'd be happy to assist if desired.
Happy cleaning!

Rick PerkinsPresidentChem LogicRick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.

If possible, assemble the board with the ECG circuits
with high impedance differential amplifiers, BGA processors and ASICs before
attaching the LGA module. Clean the board following the current assembly
process. Following the cleaning process, post bake the boards to remove any
trace or entrapped moisture. Attach the LGA module last using a no-clean flux
that requires no cleaning.

If cleaning what no-clean residue remains is
absolutely required then a precision organic base self-rinsing solvent cleaner
(not IPA) should suffice. Cybersolv 141-R has proven to be very effective
cleaning this type of flux residue.

Charlie PitarysTechnical Expert Sales SupportKyzen CorporationCharlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.

Reader Comment

Localized soldering of an LGA will be painful. The cleaning options mentioned by others should be explored. Another option to look at is an ultra-low residue/solids no-clean paste flux that leaves less than 1% residue post-reflow, as opposed to the 40-50% residue with standard no-clean fluxes. The ultra-low residue flux comes with the caveat of needing nitrogen during reflow. But if the very low flux residue levels do not affect impedance and the LGA is not subject to water cleaning, then it is worth a shot.

Karthik Vijay, Indium Corporation

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